WASHINGTON – Pollution before the first Earth Day was not only visible, it was in your face: Cleveland's Cuyahoga River caught fire. An oil spill fouled 30 miles of Southern California beaches. And thick smog choked many cities' skies.

Not anymore.

On Thursday, 40 years after that first Earth Day in 1970, smog levels nationwide have dropped by about a quarter, and lead levels in the air are down more than 90 percent. Formerly fetid lakes and burning rivers are now open to swimmers.

The challenges to the planet today are largely invisible and therefore tougher to tackle.

"To suggest that we've made progress is not to say the problem is over," said William Ruckelshaus, who in 1970 became the first head of the Environmental Protection Agency. "What we've done is shift from the very visible kinds of issues to those that are a lot more subtle today."

Issues such as climate change are less obvious to the naked eye. Since the first Earth Day, carbon dioxide levels in the air have increased by 19 percent, pushing the average annual world temperature up about 1 degree Fahrenheit, according to the National Oceanic and Atmospheric Administration.

"We've cleaned up what you can see and left everything else in limbo," said Kathleen Rogers, president of the Earth Day Network.

Improvements took shape in the form of the Clean Air Act, the Clean Water Act and changes in the way businesses treat the environment, said Denis Hayes. Those reforms, he added, grew out of the first Earth Day, an event Hayes helped coordinate."It is the most powerful, sweeping, society-wide change America has had since the New Deal," Hayes said. "The air is cleaner despite the fact that we have twice as many vehicles traveling twice as many miles."

Nancy Sutley, head of the White House Council on Environmental Quality, said progress in the past 40 years is about more than just laws. It's also about innovation that made cleaner cars. And that innovation, Sutley said, "is going to be the answer for tackling climate change."

No place illustrates progress more than the Cuyahoga River.Cleveland's main river used to periodically catch fire. On June 22, 1969, trash and an oil slick ignited. The river burned for half an hour, drawing national attention to water pollution nationwide.

People didn't swim in the river at the time, and anyone who fell in needed to be checked by a doctor.

"The river bubbled like a cauldron. There were all kinds of chemicals in there, and that was what was bubbling at the bottom," said Wayne Bratton, a boat captain then and now, and the first president of the Cleveland Harbor Conservation Committee.

On Tuesday, Wayne Bratton was aboard his boat, The Holiday. He looked over the starboard side at Collision Bend and described by telephone what he saw: "I'm looking at a lot of gulls, there's a loon, a lot of black heron."

People now fish in the river, which holds 60 species. There's a spiffy amphitheater on the river bank, which never would have been built when the water had a dreadful stench, Bratton said.It's not just the Cuyahoga. In 1957, the Public Health Service declared the Potomac River unsafe for swimming. Now Rogers lets her children swim in it.

"I don't even wash them off any more," she said.

In Los Angeles in the 1960s and 1970s, the joke was that if you moved in during the summer you wouldn't notice the nearby mountains until the winter. Now peak smog levels are only one-third as high as 40 years ago, he said.

"Unfortunately, it leads some people to think that we don't have a problem any more," said Sam Atwood, spokesman for the South Coast Air Quality Management District.

The region still has 6,000 yearly premature deaths linked to unseen tiny particles in the air that cause heart and lung problems, Atwood said.

In 1970, Ruckelshaus said, about 85 percent of pollution was from places like factories or power plants that the government could regulate. Now such sites account for only 15 percent, with most pollution coming from sources like farms that are harder to control.

That makes fixing the remaining problems politically difficult, said Russell Train, chief environmental adviser in 1970 to President Richard Nixon.

"Back in the '70s, people felt the threat of environmental mistakes and misbehavior," Train said. "There was a real threat to your health and people knew that. Today, people will accept that as a general principal, but don't feel any immediate threat from climate change or indirect source pollution from farmers."Last month was the hottest March on record worldwide. It was 1.4 degrees warmer than March 1970, according to NOAA.The average temperatures for the last 40 years are higher than the rest of the 130 years of record-keeping, said Deke Arndt, head of climate monitoring at NOAA's National Climate Data Center.

And, this week, German scientists published an analysis in the scientific journal Nature that says the greenhouse gas agreement reached by some international leaders last December in Copenhagen would lead to a 10 to 20 percent increase in carbon dioxide levels in 2020.

That puts "in dire peril" chances for limiting the effects of warming, the researchers said.

Still, the White House's Sutley is optimistic.

"The Cuyahoga River is not on fire anymore, and air quality in Los Angeles is not as bad as it was 40 years ago. I think people get those connections," Sutley said. "People get that something is changing about our climate."

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I was in L.A. recently for the first time in 10 years, and I couldn't believe how clear the skies were. That chemical taste in the air was gone, as well as that dingy brown haze, and I was surprised that I could actually see stars at night. Overall the city looked cleaner and felt safer.

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"The biodiversity crisis, the climate crisis, the deforestation crisis: we are living in an age when environmental issues have moved from regional problems to global ones. A generation or two before ours and one might speak of saving the beauty of Northern California; conserving a single species — say the white rhino — from extinction; or preserving an ecological region like the Amazon. That was a different age.

"Today we speak of preserving world biodiversity, of saving the 'lungs of the planet', of mitigating global climate change. No longer are humans overreaching in just one region, but we are overreaching the whole planet, stretching ecological systems to a breaking point. While we are aware of the issues that threaten the well-being of life on this planet, including our own, how are we progressing on solutions?

"The answer is simple: we — the human species — are failing on every major environmental problem, including those I highlight below: biodiversity, oceans, deforestation, food and water, population and consumption, and climate change. Our inability thus far to even begin solving these problems is bankrupting our Earth and will leave our children a very different — I venture to say lonelier and more chaotic — world."